Conventionally, for example, as disclosed in Japanese Laid-Open Patent Application 4-121260, a brake force control apparatus which generates, when an emergency braking is required, a brake force greater than that generated in a normal time is known. The above-mentioned conventional apparatus comprises a brake booster that generates a pressing force having a predetermined power ratio with respect to a brake pressing force FP. The pressing force of the brake booster is transmitted to a master cylinder. The master cylinder generates a master cylinder pressure PM/C corresponding to the pressing force of the brake booster, that is, the brake pressing force FP.
Additionally, the above-mentioned conventional apparatus comprises a fluid pressure generating mechanism that generates an assist hydraulic pressure by a pump as a fluid pressure source. The fluid pressure generating mechanism generates the assist hydraulic pressure which corresponds to a drive signal provided by a control circuit. The control circuit determines that an emergency braking operation is performed by a driver when a brake pedal is operated at a speed exceeding a predetermined speed, and outputs to the fluid pressure generating mechanism a drive signal which requires a maximum assist hydraulic pressure. The assist hydraulic pressure generated in the fluid pressure generating mechanism is provided to a change valve together with the master cylinder pressure PM/C. The change valve provides one of the assist hydraulic pressure generated by the fluid pressure generating mechanism and the master cylinder pressure PM/C which is higher than the other to a wheel cylinder.
According to the above-mentioned apparatus, when the brake pedal is operated at a speed smaller than the predetermined operational speed, the master cylinder pressure PM/C which is adjusted to a fluid pressure corresponding to the brake pressing force FP is provided to the wheel cylinder. Hereinafter, a control for establishing this state is referred to as a normal control. Additionally, when the brake pedal is operated at a speed exceeding the predetermined operational speed, the assist hydraulic pressure, which is a high pressure generated by the pump as a fluid pressure source, is provided to the wheel cylinder. Hereinafter, a control for increasing the fluid pressure provided to the wheel cylinder in accordance with the operational state of the brake pedal by a driver is referred to as a brake assist control. Thus, according to the above-mentioned conventional apparatus, a brake force can be controlled to a degree corresponding to the brake pressing force F P in a normal time, and the brake force can be immediately and rapidly increased after an emergency braking operation is detected.
However, the brake pedal may be pressed at a high speed despite that a driver does not intend to perform an emergency braking operation when the vehicle is moving on a rough road or a vehicle overrides a step in addition to a case where the driver intends to perform an emergency braking operation. The above-mentioned conventional apparatus always determines that an emergency braking operation is being performed when an operational speed of the brake pedal exceeds the predetermined value. Accordingly, the above-mentioned conventional apparatus has a characteristic that an emergency braking operation tends to be erroneously detected when the vehicle moves on a rough road or passes a step.